Filter-changing system for molten plastic

ABSTRACT

A filter assembly used in combination with an extruder producing a stream of pressurized molten plastic has an inlet plate formed with a Y-shaped inlet passage having a substantially circular-section upstream end adapted to receive the stream of pressurized molten plastic from the extruder, The inlet passage branches downstream of its upstream end and has a pair of downstream ends opening at substantially circular downstream ports. An outlet plate fixed immediately downstream of the inlet plate is formed with a Y-shaped outlet passage having a substantially circular-section downstream end, The outlet passage is branched symmetrically to the inlet passage upstream of its downstream end and has a pair of upstream ends opening at substantially circular upstream ports aligned with the downstream ports. A rotatable filter disk engaged between the plates is formed with a circularly annular array of substantially circular-section throughgoing holes each defining a respective substantially circular seat, The ports and holes are laterally spaced from each other such that in predetermined positions of the disk the ports open into two adjacent holes, A drive displaces the disk between respective angularly offset positions in each of which two of the holes are aligned between the ports for flow through them from the upstream ports to the downstream ports, A respective substantially circular filter unit is provided in each of the seats.

FIELD OF THE INVENTION

The present invention relates to an injection-molding system. Moreparticularly this invention concerns a system for filtering amolten-plastic stream and for changing the filter medium withoutinterrupting flow in the stream.

BACKGROUND OF THE INVENTION

A filter assembly for a plastic-molding apparatus typically has an inletplate formed with an inlet passage having an upstream end adapted toreceive the stream of pressurized molten plastic from the extruder and adownstream end opening at downstream port, an outlet plate formed withan outlet passage having a downstream end and an upstream end opening atan upstream ports aligned with the downstream port. A rotatable filterdisk engaged between the plates is formed with a circularly annulararray of throughgoing holes each defining a respective seat and theports and holes being laterally spaced from each other such that inpredetermined positions of the disk the ports open into two adjacentholes. Each seat holds a respective filter unit normally comprises of aperforated plate having an upstream side covered by a filter medium,such as a mesh or cloth.

With such a system the filter disk can be rotated to move one filterunit out of the flow path and another into the flow path withoutinterrupting the flow. The advantage of this is not only that flow cancontinue unabated, but that the pressure difference before and after thechangeover will be nominal. Since, presuming that filters are switchedat regular intervals, the filter unit that is moved out has been inservice twice as long as the one remaining in service, the change inback pressure across the filter units will not change as drastically asif clogged filter units were changed with clean ones.

As a rule the filter units are generally kidney-shaped so that they cannest closely with one another on the filter disk, only separated by thinwebs. The disadvantage of this construction is that the invariablycircular-section passages and ports do not actually cover whole filterunits, so flow through them is invariably partial. Furthermore theconstruction of both the filter disk and of the filter units themselvesis fairly complex due to the irregular shapes of the units and the seatsthey must fit in. As a result the filter assembly is an expensive pieceof equipment.

While it is known from the in-house publication"Siebwechseleinrichtungen zur Verarbeitung von Thermoplasten"(sieve-changing devices for working on thermoplastics) of MachinenfabrikJoachim Kreyenborg & Co. of Munster, Germany, to have a branched passagesystem, in this arrangement flow is switched from one branch to theother during filter changeover. Thus flow is at least part of the timeonly through one filter unit, resulting in wide pressure fluctuations.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved filter assembly for a molten plastic stream.

Another object is the provision of such an improved filter assembly fora molten plastic stream which overcomes the above-given disadvantages,that is which is of relatively simple construction and which ensuresaccurate flow through all of the filter units positioned in the flowstream.

A further object is to provide such an improved system whose filterunits can be changed easily and a method of operating the system.

SUMMARY OF THE INVENTION

A filter assembly used in combination with an extruder producing astream of pressurized molten plastic has according to the invention aninlet plate formed with a Y-shaped inlet passage having a substantiallycircular-section upstream end adapted to receive the stream ofpressurized molten plastic from the extruder. The inlet passage branchesdownstream of its upstream end and has a pair of downstream ends openingat substantially circular downstream ports. An outlet plate is formedwith a Y-shaped outlet passage having a substantially circular-sectiondownstream end. The outlet passage is branched upstream of itsdownstream end and has a pair of upstream ends opening at substantiallycircular upstream ports aligned with the downstream ports. A rotatablefilter disk engaged between the plates is formed with a circularlyannular array of substantially circular-section throughgoing holes eachdefining a respective substantially circular seat. The ports and holesare laterally spaced from each other such that in predeterminedpositions of the disk the ports open into two adjacent holes. A drivedisplaces the disk between respective angularly offset positions in eachof which two of the holes are aligned between the ports for flow throughthem from the upstream ports to the downstream ports. A respectivesubstantially circular filter unit is provided in each of the holes.

Thus this arrangement uses inexpensive circular filter units eachcomprised of a perforated disk having an upstream face provided with alayer of filter medium. Similarly the filter disk is formed with aplurality of stepped cylindrical holes receiving the filter disks sothat it can also be produced at relatively low cost. The inlet andoutlet plates are formed with symmetrical passages that respectivelyhave substantially cylindrical upstream and downstream end portionsopening at the respective upstream and downstream ends. Thus theseplates can also be made in a relatively simple process. Thus the filterassembly according to the invention can be of overall relativelyinexpensive construction. Even so, flow through the entire usablesurface areas of the filter units is certain, for efficient use of them.

According to the invention, the filter-disk holes are spaced apart by apredetermined angular distance and the upstream ports are spaced fromeach other by the same distance. The downstream ports are aligned withthe respective upstream ports. Furthermore, the ports and filter unitsare of generally the same diameter and the upstream ports are spacedfrom each other by a distance equal to less than one-quarter of thediameter. Thus the molten-plastic stream is not deflected too widely.

In accordance with a further feature of the invention the filter unitshave a thickness that is substantially less than a thickness of thefilter disk and have upstream and downstream faces spaced downstream andupstream from respective upstream and downstream faces of the filterdisk. The drive includes means for angularly indexing the disk throughangular steps of a predetermined angular displacement corresponding to awhole-number divisor of the angular spacing between adjacent filterunits in the disk. Normally this is achieved by providing the outerperiphery of the disk with a number of teeth that is twice the number offilter units so that two indexing operations are needed to move a newfilter into the flow path and an old filter out of it. The inlet andoutlet plates only axially overlap about half of the filter disk andleave the other half of the filter disk exposed so that filter units canbe serviced and worked on in the exposed half of the disk.

The filter assembly further has according to the invention means in atleast one of the plates downstream in the direction from the ports forevacuating molten plastic from a filter unit downstream of the ports,and means in at least one of the plates upstream in the direction fromthe ports for filling molten plastic into a filter unit upstream of theports. Thus air is not introduced into the stream when a new filter unitis inserted and when an old one is pulled out it is drained first.

The filter assembly according to the invention can be operated bynormally aligning a respective one of the filter units with each of theupstream ports for flow from the upstream ports only through the twofilter units aligned therewith and into the downstream ports.Alternately the filter units have a thickness that is substantially lessthan a thickness of the filter disk and have upstream and downstreamfaces spaced downstream and upstream from respective upstream anddownstream faces of the filter disk in which case the assembly isoperated by normally aligning three of the filter units with theupstream ports so that each of the upstream ports overlaps two of thefilter units and one of the filter units receives flow from both of theupstream ports.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is an end view of the system of this invention;

FIG. 2 is a partly diagrammatic section taken along line II--II of FIG.1 but with the filter disk displaced angularly for clarity of view;

FIG. 3 is a large-scale section taken along line III--III of FIG. 1;

FIGS. 4 and 5 are mainly schematic end views illustrating operation ofanother filter assembly of this invention; and

FIGS. 6 and 7 are sections like FIG. 3 showing the structure in thepositions of respective FIGS. 4 and 5.

SPECIFIC DESCRIPTION

As seen in FIGS. 1 through 3 a pair of normally stationary plates 1 and2 sandwich a spacer disk 6 and plate 7, with bolts 3, 4, 5 holding theelements together. A circular filter disk 8 can rotate between theplates 1 and 2 about an axis A of the disk 6 and is formed with eightaxially throughgoing circular seat holes 9 angularly equispaced aboutthe axis A and all lying on a circle 10 centered on the axis A. Eachhole 9 is stepped and is adapted to receive a filter ring 11 formed on aperforated filter disk 12 having an upstream face (relative to a normalflow direction D) supporting a layer of filter medium 13.

The spacer plate 7 is formed with an extension 14 on which is mounted adouble-acting hydraulic cylinder 15 whose piston rod carries anentrainment pin 16 that can catch in any of sixteen angularly equispacedentrainment teeth or hooks 19 formed on the outer periphery of the wheel8. Thus each in-and-out actuation of the cylinder 15 will angularlyindex the wheel 8 through one-sixteenth of a revolution, that is halfthe center spacing between adjacent holes 9.

As seen in FIG. 3, the upstream plate 1 is formed with a Y-shaped intakepassage 17 having a circular upstream end and branched at a dividing web20 to have two downstream ports 25 and 26 also of circular shape. Theupstream portion of the passage 17 is of cylindrical shape and then theflow cross section increases uniformly into two circular-sectiondownstream branches each terminating at a respective one of the ports 25and 26. The downstream plate 2 is formed with a complementarilyidentical outlet passage 18. All inner surfaces of the passages 17 and18 are gently rounded and the webs 20 are very small so that the splitflow moving along the passages 17 and 18 will not have to be deflectedtoo much to the side.

FIGS. 4 and 6 show how in one operational mode of the filter assembly ofthis invention two filter units shown at 21 and 22 and each comprised ofthe elements 11, 12, and 13 are aligned directly with the ports 25 and26. Thus incoming flow will start out together and then split as shownby arrows 28 and 29 in FIG. 6. Downstream of the filters 21 and 22 thesplit flows will reunite and flow out through the passage 18. When thefilter 21 starts becoming excessively clogged, the actuator 15 isoperated twice to move the unit 21 out of the flow path, to advance theunit 22 to alignment with the port 25, and to move the next unit 23 intoalignment with the port 26.

Alternately as shown in FIGS. 5 and 7 the three units 21, 22, and 23 canbe set to align with the two ports 25 and 26. In this case as is clearlyillustrated the upstream faces of all the filter units are recessedsomewhat downstream of an upstream face 30 of the filter plate 8, sothat flow will pass through all three of the plates as indicated byarrows 28 and 29. Changeover is done here as in the arrangement of FIGS.4 and 6 by two indexings of the cylinder 15 to bring into action anotherunit 24 and take out the unit 21. The advantage of this system is thatonly one of three filter units is being changed at any one time, so thatthe pressure change as filter units are changed is minimized. Inaddition during normal operation the total surface area of three unitsis available for the stream of resin, greatly reducing the resistance toflow through the filter.

FIG. 1 illustrates how the plates 1 and 2 are formed upstream relativeto a normal rotation-direction d of the disk 8 with a bottom fill port32 and a top vent port 33, both opening into a region 31 where the nextfilter unit to be used will be. Thus this filter can be filled withmolten plastic before being brought into use to prevent introduction ofair into the flow as described in my copending application 08/050,914filed Apr. 20, 1993. In addition a downstream location 34 is providedwith a bottom drain passage 35 and a top blowout line 36 connected to asource of compressed air for draining out the liquid in a filter takenout of service. Thus when the filters reach the service/exchangelocation outside of the device, where two full filters are exposed, theyare empty.

I claim:
 1. In combination with an extruder producing a stream ofpressurized molten plastic, a filter assembly comprising:an inlet plateformed with a Y-shaped inlet passage having a substantiallycircular-section upstream end adapted to receive the stream ofpressurized molten plastic from the extruder, the inlet passagebranching downstream of its upstream end and having a pair of downstreamends opening at substantially circular downstream ports; an outlet platefixed spacedly downstream of the inlet plate and formed with a Y-shapedoutlet passage having a substantially circular-section downstream end,the outlet passage being branched upstream of its downstream end andhaving a pair of upstream ends opening at substantially circularupstream ports aligned with the downstream ports; a rotatable filterdisk engaged between the plates and formed with a circularly annulararray of substantially circular-section throughgoing holes each defininga respective substantially circular seat, the ports and holes beinglaterally spaced from each other such that in predetermined positions ofthe disk the ports open into two adjacent holes; drive means fordisplacing the disk between respective angularly offset positions ineach of which two of the holes are aligned between the ports for flowthrough them from the upstream ports to the downstream ports; and arespective substantially circular filter unit in each of the holes. 2.The filter assembly defined in claim 1 wherein each filter unitcomprises a perforated disk having an upstream face provided with alayer of filter medium.
 3. The filter assembly defined in claim 1wherein the upstream and downstream passages respectively havesubstantially cylindrical upstream and downstream end portions openingat the respective upstream and downstream ends.
 4. The filter assemblydefined in claim 1 wherein the filter-disk holes are spaced apart by apredetermined angular distance, the upstream ports being spaced fromeach other by the same distance, the downstream ports being aligned withthe respective upstream ports.
 5. The filter assembly defined in claim 4wherein the ports and filter units are of generally the same diameterand the upstream ports are spaced from each other by a distance equal toless than one-quarter of the diameter.
 6. The filter assembly defined inclaim 1 wherein the filter units have a thickness that is substantiallyless than a thickness of the filter disk and have upstream anddownstream faces spaced downstream and upstream from respective upstreamand downstream faces of the filter disk.
 7. The filter assembly definedin claim 1 wherein the drive means includes means for angularly indexingthe disk through angular steps of a predetermined angular displacementcorresponding to a whole-number divisor of the angular spacing betweenadjacent filter units in the disk.
 8. The filter assembly defined inclaim 1 wherein the inlet and outlet plates only axially overlap abouthalf of the filter disk and leave the other half of the filter diskexposed, whereby filter units can be serviced and worked on in theexposed half of the disk.
 9. The filter assembly defined in claim 1,further comprising:means in at least one of the plates downstream in thedirection from the ports for evacuating molten plastic from a filterunit downstream of the ports; and means in at least one of the platesupstream in the direction from the ports for filling molten plastic intoa filter unit upstream of the ports.